The cue for disjunctive and conjugate eye movements.

The principles of control theory have been extensively applied to vision and have aided the understanding of the pupillary light reflex (Stark and Sherman, I957), accommodationtitative analyses were carried out, and mathematical models suggested which attempted to explain performance. Although some work has been done on convergence movements (Westheimer and Mitchell, I 956; Rashbass and Westheimer, I 961 a), this system has so far evaded representation in control engineering terms. If an object moves in a horizontal plane across a subject's field of vision, the eyes are called upon to make a parallel or, as it is sometimes called, conjugate movement, in order that the image may continue to fall on the fovea of each eye. Both eyes move together in the same direction. The other basic movement in the horizontal plane is a disjunctive or convergence movement. In this case the eyes rotate inwards as an object is brought nearer. A mathematical analysis was carried out (Fender and Nye, I96I) into conjugate eye movements, based on the fact that disparity of the image from the fovea is the cue for an eye movement. The disparity-actuated negative feedback system operates so as to eliminate error between fovea and image. If the image of the object is shifted through an angle 0 with respect to the eye, then the image is displaced RO on the retina away from the fovea, R being the distance between the retina and the eye's centre of rotation. Since the need to fix an image on the fovea is also the cue for convergence (Alpern, i962, p. 99), an equation similar to that proposed by Fender and Nye should also be applicable to this system. Fig. I shows an unambiguous system of error detection which it is proposed that the eye employs for conjugate and disjunctive movements. A signal proportional to the distance P is taken and is added to a corresponding signal from the other eye proportional to Q. Q and P are simply the distances in each eye between the fovea and the image (or feature of maximum interest in a large image). A signal proportional to P-Q is the cue for a convergence movement. When a pure convergence movement is to be executed, the value of P + Q, the cue for a conjugate movement, is zero. Likewise P-Qis zero when a pure conjugate movement is called for. On this basis the eye …